1. Field of the Invention
The present invention relates to an acidic Snxe2x80x94Cu alloy plating bath. More particularly, the present invention relates to an acidic Snxe2x80x94Cu alloy plating bath which can prevent the Cu substitution-deposition problem which is a serious problem in large scale industrial production.
2. Description of Prior Art
Conventionally, Snxe2x80x94Cu alloy plating has been applied to bronze plating for decorating purposes. In recent years, however, the Snxe2x80x94Cu alloy plating is attracting attention as a plating method which can be used in place of solder plating (Snxe2x80x94Pb alloy plating).
As Snxe2x80x94Cu alloy plating baths, a copper cyanide-alkaline stannate bath (Japanese Patent Application Laid-open No. 96936/1977), a pyrophosphoric acid bath (Japanese Patent Applications Laid-open No. 72196/1981 and No. 272394/1986), and a copper cyanide-copper pyrophosphate bath (Japanese Patent Application Laid-open No. 60091/1982) are known. In addition, an Snxe2x80x94Cu alloy plating bath using an inorganic acid in an amount sufficient to maintain 2.0 or less pH has been disclosed as an acidic bath (Japanese Patent Application Laid-open No. 177987/1982).
Of these baths, the acidic bath appears to be more advantageous than other baths in industrial application due to the excellent current efficiency. However, actually the bath has a serious problem which renders the method difficult to be applied. Specifically, a problem with the acidic Snxe2x80x94Cu alloy plating bath, which is not mentioned in the Japanese Patent Application Laid-open No. 177987/1982, is that copper deposits due to substitution when the plated material is made of a metal with an electric potential lower than copper such as iron and nickel. This is caused due to the presence of free Cu+2 ion in the acidic Snxe2x80x94Cu alloy plating bath and unduly impairs plating adhesion. Copper deposits also on the anode when Sn is used as the material of the anode, rendering the anode difficult to be dissolved and interfering with the plating bath stability. Moreover, SnO2 and the like cause turbidity if Sn2+ and Cu2+ are present at the same time.
Use of a complexing agent may be one way of stabilizing Cu2+ to overcome this type of problems. A complexing agent, however, impairs waste water processability, thus creating another problem of making processing of waste water difficult.
The subject to be solved in the present invention is therefore to provide an acidic Snxe2x80x94Cu alloy plating bath which can prevent the Cu substitution-deposition problem and turbidity due to production of SnO2 without using a complexing agent.
The inventors of the present invention have conducted extensive studies to solve the above-described problem and have found that the Cu substitution-deposition problem and turbidity of the plating solution can be prevented without using a complexing agent by adding a thiourea compound to an acidic solution comprising Sn, Cu, and an acid such as analkane sulfonic acid, alkanol sulfonic acid, sulfuric acid, or the like in an amount sufficient to solve Sn and Cu.
Specifically, an object of the present invention is to provide an acidic Snxe2x80x94Cu alloy plating bath composition comprising: (a) Sn2+ ions and Cu ions, (b) at least one acid selected from the group consisting of alkane sulfonic acids, alkanol sulfonic acids, and sulfuric acid, and (c) a thiourea compound.
Another object of the present invention is to provide an acidic Snxe2x80x94Cu alloy plating bath composition comprising a nonionic surfactant in addition to the above-described components (a) to (c).
Still another object of the present invention is to provide an Snxe2x80x94Cu alloy plating method comprising subjecting a material to be plated to cathodic electrolysis in either of the above-mentioned acidic Snxe2x80x94Cu alloy plating bath compositions.
Other objects, features and advantages of the invention will hereinafter become more readily apparent from the following description.
The acidic Snxe2x80x94Cu alloy plating bath composition of the present invention (hereinafter called xe2x80x9cSnxe2x80x94Cu plating bathxe2x80x9d) comprises (a) Sn2+ ions and Cu2+ ions, (b) at least one acid selected from the group consisting of alkane sulfonic acids, alkanol sulfonic acids, and sulfuric acid, and (c) a thiourea compound.
Of these components, the Sn2+ ions and Cu2+ ions which are the component (a) are made available at the initial time of preparation by providing oxides of these ions, such as tin (II) oxide and copper (II) oxide, and adding an anion of the component (b) to the oxides, or by dissolving the anion salt of the component (b) of Sn2+ion or Cu2+ion, for example, tin methane sulfonate, copper methane sulfonate, tin ethane sulfonate, copper ethane sulfonate, tin isopropanol sulfonate, copper isopropanol sulfonate, tin sulfate, or copper sulfate. During plating, the Sn2+ ions and Cu2+ ions are made available by the anode or the anion salt of Sn2+ion or Cu2+ion.
On the other hand, the acid selected from the group consisting of alkane sulfonic acids, alkanol sulfonic acids, and sulfuric acid, which is the component (b), is supplied as a free acid or as a salt of Sn2+ion or Cu2+ion which is the component (a).
As the thiourea compound which is the component (c), thiourea, diethyl thiourea, phenyl thiourea, allyl thiourea, acetyl thiourea, diphenyl thiourea, benzoyl thiourea, and the like can be given.
The respective amount of Sn2+ ions and Cu2+ ions contained in the Snxe2x80x94Cu plating bath of the present invention as component (a) is preferably 0.5-20 wt % and 0.01-2 wt %, and particularly preferably 1-5 wt % and 0.02-0.2 wt %. The ratio of Sn2+ ions and Cu2+ ions may be varied according to the composition of the target alloy plating to be deposited. For instance, when an eutectic alloy plating consisting of about 99.3% of Sn and about 0.7% of Cu is desired, the ratio of Sn2+ ions and Cu2+ ions may be about 50:1 to 100:1.
The amount of alkane sulfonic acids, alkanol sulfonic acids, and sulfuric acid contained in the Snxe2x80x94Cu plating bath as component (b) may be about 5-300 g/l, and preferably about 50-150 g/l.
The amount of a thiourea compound contained in the Snxe2x80x94Cu plating bath as component (c) may be about 0.1-20 g/l, and preferably about 1-10 g/l. Because the thiourea compound has an action of interfering with deposition of Cu, the amount to be added is preferably increased when the Snxe2x80x94Cu plating bath contains a large amount of Cu2+ ions.
In addition to the above-described essential components, a nonionic surfactant may be added to the Snxe2x80x94Cu plating bath as component (d). As specific examples of the component (d), surfactants containing any one of the compounds shown by the following formulas (1) to (4) as a major component can be given.
[Chemical Formula 1]
wherein R1 represents a hydrogen atom or a residue obtained by excluding a hydrogen atom from the hydroxyl group of an aliphatic alcohol having 8-22 carbon atoms, phenol substituted with an alkyl group having 1-25 carbon atoms, xcex2-naphtol substituted with an alkyl group having 1-25 carbon atoms, alkoxylated phosphoric acid having 1-25 carbon atoms, sorbitan estrified with a fatty acid having 8-22 carbon atoms, or styrenated phenol in which the hydrogen atom may be substituted by an alkyl group having 1-4 carbon atoms or a phenyl group, R2 represents an alkyl group having 8-18 carbon atoms, R3 and R4 individually represent a hydrogen atom or an alkyl group having 1-5 carbon atoms, A represents xe2x80x94CH2CH2Oxe2x80x94, B represents xe2x80x94CH2CH(CH3)Oxe2x80x94, m1 and n1 are individually an integer from 0 to 30, m2, n2, m3, and n3 are individually an integer from 0 to 40, and m4 and n4 are individually an integer from 0 to 20, provided that m1 and n1, m2 and n2, m3 and n3, or m4 and n4 are not simultaneously zero, m1 to m4 and n1 to n4 individually indicate the total number of substituents, and positions of A and B are not limited.
All these nonionic surfactants can be prepared by adding prescribed mols of ethylene oxide and/or propylene oxide to a corresponding aliphatic alcohol, substituted phenol, alkyl substituted xcex2-naphtol, alkoxyl phosphoric acid, estrified sorbitan, styrenated phenol, ethylenediamine, monoalkyl amine, or diphenol which may be substituted by an alkyl. In addition, these nonionic surfactants are readily available as commercial products.
Given as examples of commercially available products are Plurafac LF401 (manufactured by BASF) having the chemical structure of the formula (1), Tetronic TR-702 (manufactured by Asahi Denka Kogyo Co., Ltd.) having the chemical structure of the formula (2), Naimeen L-207 (manufactured by Nippon Oil and Fats Co., Ltd.) having the chemical structure of the formula (3), Liponox NC-100 (Lion Co., Ltd.) having the chemical structure of the formula (4), and the like. The nonionic surfactant is generally added to the Snxe2x80x94Cu plating bath to make a concentration of about 0.5-30 g/l.
Metal ions other than Sn and Cu may further be added to the Snxe2x80x94Cu plating bath composition of the present invention, in which case the plating bath composition is a ternary alloy or more metal-containing alloy composition containing tin and copper as major components. Bi3+, Ag+, and the like can be given as examples of metal ions which can be added.
To perform Snxe2x80x94Cu alloy plating using the Snxe2x80x94Cu plating bath of the present invention, the object to be plated is immersed in the plating bath prepared as mentioned above and electrolysis is carried out using that object as a cathode. Specifically, electrolysis is carried out using metallic tin, tin-copper alloy, or the like as an anode, stirring by cathode rock, jet or the like under the conditions of a temperature of about 10-50xc2x0 C. and a current density of about 0.1-100 A/dm2.
Because Cu substitution-deposition is suppressed due to the action of a thiourea compound (component (c)) in the Snxe2x80x94Cu plating bath of the present invention, Snxe2x80x94Cu alloy can be plated onto the material made of a metal with an electric potential lower than copper such as iron and nickel without impairing plating adhesion.
Moreover, since the thiourea compound suppresses the action of Cu2+, Sn2+ is difficult to be oxidized so that turbidity due to SnO2 can be prevented.
Other objects, features and advantages of the invention will hereinafter become more readily apparent from the following description.
The present invention will be described in more detail by way of Test Examples and Examples which should not be construed as limiting the present invention.
 less than Measurement of Deposited Cu onto Anode greater than 
The following two acidic Snxe2x80x94Cu alloy plating bath compositions were prepared. A metallic Sn anode was immersed in each of the compositions. After 24 hours, the anodes were removed from the compositions to visually inspect the surface conditions. In addition, Cu concentrations before immersing the anode and after removing the anode were analyzed by the atomic absorption spectroscopy. Results are shown in Table 1.
 less than Acidic Snxe2x80x94Cu Alloy Plating Bath Composition greater than 
Composition (1): Base solution (Sn2+: 20 g/l, Cu2+: 1 g/l, methanesulfonic acid: 150 g/l)
Composition (2): Base solution+thiourea 6 g/l
 less than Results greater than 
As clear from Table 1, Cu was seen depositing onto the anode in the composition (1). The analysis using the atomic absorption spectroscopy revealed a significant decrease in the concentration of Cu in the composition (1), confirming that Cu deposited onto the anode. In contrast, Cu did not deposit onto the anode in the composition (2).